What resolution do photos need for a printed album? (the dpi maths)

Printers lay down dots, and dpi (dots per inch) is how many of your image's pixels are used per inch of paper. The number of pixels a photo needs is simply the printed size in inches multiplied by the dpi. A slot 4 inches wide at 300 dpi needs 1200 pixels across; at 200 dpi it needs 800. Everything else about print resolution follows from that.

300 dpi is the commercial-print standard and the point beyond which the eye cannot see improvement at reading distance. 200 dpi is visibly fine for photos in a booklet held in the hand, and is what Foldpic's free projects export at; paid projects export at 300 dpi. Below about 150 dpi photos begin to look soft, and below 100 dpi they look blocky.

Here are the numbers for the pages Foldpic makes. A full-page photo needs the full figure; a photo in a half-page slot needs roughly half the width, and so on.

A 12-megapixel phone photo is about 4032 by 3024 pixels, enough for an A5 page at 300 dpi with room to crop. Even a 2015 phone at 8 megapixels (3264 by 2448) is comfortably above the A5 figure. Camera JPEGs and RAW exports are larger still. Resolution is almost never the problem with photos you took yourself.

The problem images come from elsewhere. Screenshots are the size of your screen, often 1170 pixels wide or less, so they only work in small slots. Photos received through WhatsApp, Messenger or most social apps have been resized and recompressed, typically to around 1600 pixels on the long side and heavily compressed, which prints soft at full page. Ask the sender for the original, or use “send as document” in messaging apps, which skips the compression.

Foldpic's review screen compares each photo's pixel size with the slot it is in at the export dpi and marks any that fall short with a “will print soft” warning. It is a warning, not a block: a slightly soft photo of a moment that matters is better than no photo. The fix is one of three things: swap in a higher-resolution version, move the photo to a smaller slot, or crop less of it away, since cropping throws away pixels.

Note that upscaling in an editing app does not add detail; it just makes a bigger soft image. If the original is small, use it small.

Two photos with the same pixel count can print very differently. Motion blur, missed focus and heavy noise all show more on paper than on a bright screen, because paper has less contrast. When choosing between similar frames, zoom to 100 percent on screen and pick the one where the eyes are crisp. Paper also prints darker than a screen, so a photo that looks moody on your phone may print murky; lift the shadows a little before export.